In a known automatic power transmission mechanism, the gear ratios are shifted between each other by selectively actuating fluid operated friction engaging means, for example, drive clutches or brake bands. To effect shifting between the gear ratios of known hydraulic control devices or passenger or personal cars special shift valves are used having spools provided with a number of circumferential lands, wherein the position of the spool is controlled by an operating fluid distributed directly to the shifting members. As the quantity of distributed fluid increases with the transmitted power while the sensitivity of the control device decreases with the quantity of distributed fluid, such control devices cannot be used for heavy vehicles of high-power. There is known a hydraulic control device for the automatic control of power transmission of heavy vehicles which is a system employed in Allison automatic transmission model AT 540 published in Service Manual of Allison Division of General Motors, Box 894, Indianapolis, IN 46206.
These known hydraulic control devices for automatic power transmission of heavy vehicles use for shifting between two gear ratios a known shift valve, having a spool shiftable between the initial first position allowing to connect the low-speed friction engaging means with the main input pressure source while discharging actuating pressure from the high-speed friction engaging means and between alternate second position allowing to connect the high-speed friction engaging means with the main input pressure source while discharging actuating pressure from the low-speed friction engaging means. The position of the shift spool is controlled by the main input pressure supplied to the terminal work face of the outer circumferential land of the said shift spool. The main pressure acting on the terminal work face of the outer land initiates the shifting of the shift spool from initial first position to alternate second position and the high-speed friction engaging means are actuated. The shift spool is urged by a spring against the opposite acting main pressure into the initial first position and after the main pressure disappears, the shift spool returns by action of a spring to the initial first position and the low-speed friction engaging means are in this way actuated.
The main pressure acting on the terminal work face of the outer land of the shift spool is delivered through a signal pressure conduit from a known signal valve having a shiftable multi-spool provided with a number of circumferential lands, the spool forced into an initial first position by a spring, allowing in the initial first position discharging of main pressure from the signal pressure conduit and closing the input of the main pressure into the signal valve.
The multi-spool is shifted toward alternate second position by action of a vehicle speed-dependent pressure and modulated pressure, modulated pressure being formed in a separated valve as inverse proportion to the engine load-dependent pressure. In alternate second position of the multi-spool the connection of the main input pressure source through the signal valve to the shift valve is allowed. The forces resulting of the speed-dependent and load-dependent pressures acting on the spool are of the same direction opposite to the action of the spring urging the spool back toward the initial first position.
In this respect the rigidity of the said spring is to be substantial. A high rigidity of the said spring, however, increases the forces opposing the movement of the spool. The intensity of such opposing forces is variable along the spool travel and cannot be foreseen. For this reason a strong spring is undesirable.
It proved to be unsuitable to shift the signal spool by action of nothing but the spring and it became necessary to exert the speed-dependent pressure on the spool in opposite direction to the load-dependent pressure.
The present invention provides a solution to such problem,.